—Yay... (as opposed to, say, boo) is for joy and exultation. (Rhymes with nay or day.)

—Yeah.... (synonym of yes, opposite of nah) is for ordinary assent. (Like, 'Yeah, I saw that movie.' or 'Yeah, buddy.')

—Yea.... (opposite of nay) is for formal assent during a vote.

It's just that many people type 'yea' (or even just 'ya') when they mean 'yeah.' Others tend to type 'YEAH!' when they mean 'YAY!'

While we are on the subject of 'ya' — just a reminder it is y'all, not ya'll. The 'y' is short for 'you' — so just drop the 'ou' and replace with the apostrophe and add 'all' — and then y'all have that country twang goin' on.

YAY! I'm going to vote using 'yea' or 'nay' today! Now y'all have a good day, yeah?